During manufacturing of a display device, a semiconductor active layer of a TFT (Thin Film Transistor) is generally manufactured by using a CVD (Chemical Vapor Deposition) process. The CVD process is a process where mixed gases containing a material for making a film is used to chemically react on a surface of a heated substrate so as to precipitate and form the film. The CVD process generally includes PECVD (Plasma Enhanced Chemical Vapor Deposition), APCVD (Atmospheric Pressure Chemical Vapor Deposition), LPCVD (Low Pressure Chemical Vapor Deposition) or the like.
With rapid development of the display techniques, requirements for quality of the display device from customers become higher and higher. Thus, the quality of coating a film on a substrate becomes an issue of concern and research in the industry.